The present invention relates to an exposure optical system for an electrostatic copying machine or the like.
An electrostatic copying machine typically comprises an optical system which focusses a light image of an original document on a charged photoconductive member such as a rotating drum. The light image causes localized photoconduction of the drum and forms an electrostatic image thereon. The electrostatic image is developed to produce a toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the document.
Glass optical fibers have been developed and marketed under the trade name "SELFOC" which are formed in such a manner that the index of refraction thereof decreases parabolically in the radial direction from the central axis of the fibers. These optical fibers act as converging lenses. More specifically, if a first object is illuminated and an optical fiber of this type which is provided in the correct length is placed between the first object and a second object with distances from the opposite ends of the optical fiber to the first and second objects being equal, an erect full size image of the portion of the first object adjacent to the facing end of the optical fiber will be focussed on the second object.
In an electrostatic copying machine, these optical fibers are employed in a row in a very advantageous manner which results in reduction of the size of the exposure optical system. The row of optical fibers extends coextensively parallel to the axis of the photoconductive drum and the document and drum are moved relative to the opposite ends of the optical fibers at the same surface speed. The elongated, narrow beam of light focussed onto the drum in combination with the movement of the document and drum constitutes a scanning operation by which an image of the original document is formed on the drum.
The optical fiber system reduces the distance along the optical path of the light image from the document to the drum from several tens of centimeters to several centimeters, or by a factor of about 10. This contributes significantly to efficient space utilization in the copying machine and enables a more compact configuration.
However, due to the overall arrangement of the component parts in the copying machine it may be disadvantageous to provide the drum in such close proximity to the document without various parts of the scan drive system and other elements intervening. This drawback has precluded the widespread adaptation of focussing optical fiber arrays in the exposure systems of electrostatic copying machines, facsimile devices and the like.